Optical data processing system with reflective liquid crystal light valve

ABSTRACT

There is disclosed a high performance coherent optical data processing system using a reflective mode liquid crystal light valve which is particularly suited for application to real-time coherent optical data processing. A preferred example of the system uses a reflective light valve comprising a CdS photoconductor, a CdTe light absorbing layer, a dielectric mirror, and a liquid crystal layer sandwiched between indium-tin-oxide transparent electrodes deposited on optical quality glass flats. The non-coherent light image is directed onto the photoconductor; this reduces the impedance of the photoconductor, thereby switching the AC voltage that is impressed across the electrodes onto the liquid crystal to activate the device. The liquid crystal is operated in a hybrid field effect mode. It utilizes the twisted nematic effect to create a dark off-state (voltage off the liquid crystal) and the optical birefringence effect to create the bright on-state. The liquid crystal thus modulates the polarization of the coherent read-out or projection light responsively to the non-coherent image. An analyzer is used to create an intensity modulated output beam which is projected through a Fourier transform lens onto a screen or other detector means.

CROSS-REFERENCE TO RELATED COMMONLY OWNED APPLICATIONS AND PATENTS

The present invention is an improvement over the inventions shown inU.S. Pat. No. 3,744,879 issued to T. D. Beard on July 10, 1973, U.S.Pat. No. 3,824,002 issued to T. D. Beard on July 16, 1974, and over theinvention shown in the copending application of William P. Bleha, JanGrinberg, Joseph Jenney and Gary D. Myer Ser. No. 538,381 filed on Jan.6, 1975 and entitled "Photoactivated Liquid Crystal Field Effect LightValve Display Device." This invention may also use, but does notrequire, the photodetector of U.S. Pat. Application Ser. No. 625,331,now U.S. Pat. No. 3,976,361 field Oct. 22, 1975 on behalf of L. M. Fraasand W. P. Bleha Jr. entitled "Charge Storage Diode With Graded DefectDensity Photocapacitive Layer". This system uses the light valve of thecopending application Ser. No. 664,776 concurrently filed herewith bythe same inventors entitled "Reflective Liquid Crystal Light Valve withHybrid Field Effect Mode." All of the above are assigned to the assigneeof the present application.

FIELD OF THE INVENTION

This invention pertains generally to coherent optical data processingand to electro-optical phenomena in liquid crystal devices and inparticular to reflective light valve display devices which utilize suchphenomena in a hybrid field effect mode suitable for optical dataprocessing.

BACKGROUND OF THE INVENTION

The science and technology of coherent optical data processing (CODP)has existed as a recognized discipline since the early 1950's when thefollowing authors and others published the initial concepts upon whichthe field is now based. Such authors include:

1. D. Gabor, Mass. Inst. of Tech., Electronics Research Laboratory Tech.Rep. No. 238 (1952).

2. E. L. O'Neill, I.R.E. Trans. on Infor. Theory, IT-2, Pg. 56 (1956).

3. A. Blank Lapierre, Symposium on Microwave Optics, McGill Univ.Montreal, Canada, Pg. 46 (1953).

4. P. Elias, et al, J. Opt. Soc. Am. 42, Pg. 127 (1952).

5. P. M. Duffieux, L'Integrable de Fourier et ses Applications aL'Optique, Faculte des Sciences, Besancon, France (1946).

6. A. Marachal and P. Croce, Compte rendu, 237, Pg. 607 (1953).

7. L. Cutrona, et al, IRE Trans. on Infor. Theory, IT-6, Pg. 386 (1960).

The above noted U.S. Pat. No. 3,744,879 to T. D. Beard describes a morerecent implementation of a coherent optical data processor using atransmissive liquid crystal light valve as a spatial filter to controltransmissivity of coherent light through the processor. Not only is theBeard liquid crystal light valve transmissive rather than reflective,but also it relies on the dynamic scattering mode which has not provenas satisfactory in operation as the hybrid field effect mode to bedescribed herein.

The promise of CODP, from the start, has been to perform data processingin many parallel processing channels simultaneously and quickly. Thus itwas regarded to be an alternative to the serially organized electronicdigital computer, particularly suited to the processing oftwo-dimensional data bases such as photographic images or multi-channel,wide bandwidth electronic signals. Despite its obvious advantages, CODPhas not been regarded as a general purpose data processing technology.Inherently, CODP is a linear, analog process. For these reasons, andbecause the physical components and subsystems that implement theseprocesses are non-programmable, CODP is substantially less flexible thanis the electronic digital computer. As a result, CODP is bestimplemented in the form of special purpose processing hardware.

There exists many problem areas that can make use of such a specialpurpose processing capability. Unfortunately, few have heretoforebenefited from the potentials of CODP. One of the principle reasons isthat CODP, as it has heretofore been practiced, cannot be performed inreal time. Speed of data through-put is a basic requirement for aspecial purpose data processor. Yet CODP, because of its historicreliance on photographic film both for inputting data and for use as aspatial filter, has been an off-line process. This is also true of theBeard device which relies on a transparency, 60, for inputting a signalimage. Thus CODP has suffered the worst of both processing worlds -- --limited flexibility and off-line operation.

The present invention can resolve this dilemma by providing a highperformance, real time non-coherent to coherent light image convertorusing a reflective light valve. The system may accept input images inreal-time from non-coherently illuminated scenes which modulate thespatially coherent illumination of the CODP. More generally, the systemmay also be used in other image processing and projecting applicationsknown to the prior art.

SUMMARY OF THE INVENTION

The reflective mode liquid crystal light valve device used in thepresent invention is a special adaptation of the AC photoactivatedliquid crystal light valve described in U.S. Pat. No. 3,824,002 to T. D.Beard and in the above-identified copending patent application ofWilliam Bleha et al. Basically, the device consists of a sandwich ofthin films that electrically control the optical birefringence of a thin(approximately 2 micrometer) liquid crystal layer. The device has highresolution (greater than 100 lines per millimeter limiting resolution),high contrast (greater than 100 to 1), high speed (10 milliseconds, on;15 milliseconds, off) and high input sensitivity (approximately 0.3 ergsper square centimeter at threshold). Moreover, it has several practicaladvantages. It is compact (solid state), low power (several milliwatts),inexpensive to manufacture (thin film technology), and operates from asingle low voltage (5-10 volts rms) power supply. In a single device theinvention combines two effects in liquid crystal phenomena, namely therotation of the polarization state by the twisted nematic alignment inthe off-state of the device and the birefringence of the tiltedmolecules in the on-state of the device. The use of this hybrid fieldeffect mode by contrast to the dynamic scattering mode permits the useof a very thin layer of liquid crystal which results in very fastresponse which is crucial for optical data processing systems. The abovementioned hybrid field effect mode combines this high speed with highcontrast. This hybrid field effect mode is achieved by providing the twomicrometer thick liquid crystal layer with a twist angle which is lessthan the heretofore known 90° twist or spiral. This angular relationshipis determined by prefabrication of alignment means in each of the twoinsulating layers confining the liquid crystal material in such afashion that the two opposed faces have alignment directions which ifprojected to any plane between and parallel to the two faces intersectat an acute angle. The birefringence of the tilted molecules in theon-state when an electric field is applied across the liquid crystallayer may be further enhanced by illuminating the cell with an off-axisprojection beam. When the incident light angle is other thanperpendicular the effective birefringence is not zero even when thealignment is ideal homeotropic (which is defined as an alignment inwhich the long axis of the molecules is oriented perpendicular to theelectrode surfaces and which may be produced by application of anappropriate electric field). On the other hand, no significant change isobserved in the twisted nematic behavior if the incident light beamangle deviates from the normal to the liquid crystal layer by less than20 ° or 30°. Such a hybrid field effect cell using a 45° twist angle forthe twisted nematic alignment to control polarization by the rotationeffect in the off-state and using off-axis incident projection light totake advantage of the tilt of the molecules resulting in control ofpolarization state by the birefringence effect in the on-state of thedevice results in a liquid crystal cell having very good contrast andhigh speed sufficient for real time use in optical data processingsystems.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the invention willbe more fully understood from the following detailed description of apreferred exemplary embodiment taken in conjunction with theaccompanying drawings wherein like reference characters refer to likeparts throughout and in which:

FIG. 1 is substantially a reproduction of FIG. 1 in U.S. Pat. No.3,824,002 issued to Terry Beard on July 16, 1974 showing a sectionalview of a prior art light valve of this type which may be modified foruse in the present invention.

FIGS. 2a and 2b are respectively a circuit diagram showing an equivalentcircuit and a diagram of voltage waveforms of an ideal AC light valvesubstrate of the type shown in FIG. 1 in the absence of illumination.

FIGS. 3a and 3b are respectively an equivalent circuit diagram and adiagram of voltage and current waveforms of an ideal AC light valvesubstrate in the presence of illumination.

FIG. 4 is an equivalent circuit of an actual complete AC light valve.

FIG. 5a and 5b are diagramatic showings of the type of twisted nematicliquid crystal configuration used in the off state of the light valve ofFIG. 1 in accordance with the present invention.

FIGS. 6a and 6b are diagramatic views illustrating the operation of thehybrid field effect device used in the present invention in the tiltedmode occurring in the on-state of the electric field.

FIGS. 7a and 7b are graphs illustrating calculated values of moleculeorientation as a function of position across the liquid crystal layer,the graph in FIG. 7a showing values for the twist angle and the graph inFIG. 7b showing values for the tilt angle.

FIG. 8 is a graph showing experimental curves that characterize thehybrid field effect liquid crystal device. Curve A is a plot of thepercentage of light transmission as a function of applied voltage for a45° twist angle whereas curve B is the same function for a 90° twistangle.

FIG. 9 is a schematic diagram of a coherent optical data processingsystem in accordance with the present invention.

FIG. 10 is a sensitometry curve comprising a graph of percenttransmission versus input light intensity for the hybrid field effectlight valve used in the system of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

There is shown in FIG. 1 a cross-sectional view of the light valveshowing the general configuration of this device as used in the presentinvention. Actually, the showing of FIG. 1 is substantially areproduction of FIG. 1 is U.S. Pat. No. 3,824,002 issued to Terry D.Beard on July 16, 1974. The device as used in the present invention hassubstantially the same structural configuration as the Beard device, butis modified in the manner in which the alignment means for the moleculesof the nematic liquid crystals are arranged in the device and in themode of operation resulting therefrom. Also, the light valve as used inthe present invention uses a positive dielectric anisotropy in contrastto the negative dielectric anisotropy nematic liquid crystals used inthe Beard device. This makes the change of alignment possible.

The complete description of the structure of the device of FIG. 1 andits mode of operation in the dynamic scattering mode is set forth in theBeard patent. Briefly, it will be noted that the AC light valve 15consists of a number of thin film layers sandwiched between two glasssubstrates 1 and 1a. A low voltage (5 to 10 volts rms) audio frequencypower supply 16 is connected by leads 12 and 12a through switch 14 tothe outer, thin film indium-tin-oxide (ITO) transparent electrodes 2 and2a. Thus the power supply is connected across the entire thin filmsandwich. The cadmium sulfide photoconductor layer 7 and the lightblocking cadmium telluride layer 6 combine to create a rectifyingheterojunction. The dielectric mirror 5 and the blocking layer 6separate the photoconductor 7 from the read-out light beam 9. This is amajor design feature of the AC light valve. It enables simultaneouswriting by writing or input light beam 10 and reading of the device bybeam 9 without regard to the spectral composition of the two light beams9 and 10. Furthermore, the dielectric mirror prevents the flow of DCcurrent through the liguid crystal. This enhances the lifetime of thedevice. Finally, the dielectric mirror 5 can be designed to reflect anyportion of the visible spectrum thereby optimizing the ability of themirror to separate beams 9 and 10 and, at the same time, maximizing thereflectivity of the device. This, together with the chemically inertSiO₂ insulating layers 3 and 3a which bound the liquid crystal layer 13ensures a very long lifetime for the device. The liquid crystal used inthis device is typically a nematic material such as a biphenyl which isheld in the space defined by the two insulating layers 3 and 3a and thespacers 4 and 4a which are employed to maintain a suitable gap betweenthe insulating films.

In FIG. 2a there is shown an equivalent circuit diagram of an ideallight valve substrate of the type shown in FIG. 1. The diode 20represents the heterojunction diode between the layers 6 and 7, and thecapacitor 21 represents the capacitance of the dielectric mirror 5. Thecircuit is drawn for the case of no input illumination light to thephotoconductor. When the AC voltage power supply 16 is connected to sucha circuit, the capacitor 21 is charged to the negative peak voltage(-V_(p)) of the power supply during the first cycle by power supplyvoltage 16a shown in FIG. 2b. This voltage then serves as a backbiasvoltage on the diode for all values of the sinusoidal varying powersupply voltage. Assuming infinite back resistance for the diode, thesteady-state current flow in this circuit will be zero, independent ofthe frequency, waveform (providing it is periodic), and amplitude of thepower supply voltage. Thus there will be no current flow in thenon-illuminated resolution element of the ideal AC light valve, and thediode voltage will be as shown in waveform 20a in FIG. 2 b.

Now, let us consider what happens in an illuminated element. In the caseof the idealized circuit shown in FIG. 3a, the incident photonsintroduce a leakage resistance 22 across the diode 20; this resistancedischarges the capacitor 21 during the back-bias cycle of the diode. Theapproximate waveforms are shown in FIG. 3b. If the current is highenough, the liquid crystal in the illuminated element will be drivenabove its electro-optic threshold by the field developed across it. Thiseffect can then be read out by the projection beam.

A more realistic equivalent circuit of the AC light valve is shown inFIG. 4. In this circuit R₁ and C₁ represent the resistance andcapacitance, respectively, of the liquid crystal, C₂ the capacitance ofthe mirror, R₄, (R₃ + R₄) and C₃ represent respectively the forwardresistance, back resistance and the capacitance of the diode. Inderiving this circuit we assume that the leakage resistance of themirror, R₂, is very high which means that R₂ C₂ ω is much greater than 1where ω is the basic frequency of the power supply, so that we canneglect the influence of R₂ on the circuit. Hence R₂ is not shown.Unlike the idealized circuit, a substrate that is represented by thecircuit shown in FIG. 4 will pass current when the photoconductor is notilluminated, as well as when it is. In this circuit, the illuminationdecreases the values of R₃ and R₄, and increases the capacitance C₃ (dueto a photocapacitance effect in the junction). As a result the currentflow in the illuminated element is larger than in the nonilluminatedelement. It is this difference in the current flow that provides us withmeans for controlling the liquid crystal electro-optic effect with thephotoconductor. So, the substrate has to be designed in such a fashionthat the current flow in the nonilluminated element is less than theliquid crystal threshold level, and in the illuminated element is abovethe threshold by the desired amount. Generally speaking, our goal is tomaximize this current ratio, which we call the switching ratio of thedevice, since this ratio determines the photoelectric efficiency of thelight valve. It has been found that switching ratios vary between 1.1and 2.1 for input light intensity levels from 10 microwatts per squarecentimeter to saturation with a value of 1.8 for the switching ratio at400 microwatts per square centimeter. At these latter values theexcitation and decay times in milliseconds were 10 and 30 respectively.

The AC light valve is inherently a multi-purpose device. Its differentfunctions are realized by changing the manner in which the liquidcrystal is applied to and used in the device. Liquid crystals exhibitseveral different electro-optic effects. These include dynamicscattering and two separate field effects which are respectively calledoptical birefringence and the twisted nematic effect. Dynamic scatteringis described, for example, by G. H. Heilmeier, L. A. Zanzoni, and L. A.Barton in the proceedings of the IEEE Vol. 56 page 1162 (1968) and inthe IEEE Transactions of Electron Devices, Vol. ED-17, Page 22 (1970).The optical birefringence effect is described in the followingreferences: M. F. Schiekel and K. Fahrenschon, Applied Physics Letters,Vol. 19 page 391 (1971); F. J. Kahn, Applied Physics Letters, Vol. 20page 199 (1972); and R. A. Soref and M. J. Rafuse, Journal of AppliedPhysics, Vol. 43 page 2029 (1972). The twisted nematic effect has beendescribed in the following references: M. Schadt and W. Helfrich,Applied Physics Letters, Vol. 19 page 127 (1971); A. Boller, M. Scherrerand M. Schadt, Proceedings of the IEEE Vol. 60 page 1002 (1972).

Also, U.S. Pat. No. 3,625,591, in the name of Marvin J. Freiser and IvanHoller, entitled "Liquid Crystal Display Element," and assigned to IBMCorporation discloses a cell having a nematic liquid crystal which inits quiescent state (in the absence of an electric field) has all itsmolecules aligned parallel to a given direction along which the cell'selectrodes have been rubbed. According to the teachings of that patent,when a low voltage AC source is applied across the electrodes, theliquid crystal conducts current which produces a chaotic redistributionof the molecules of the liquid crystal film, which in turn, serves todepolarize the light. By means of cross polarizers this redistributionmay be made visible to an observer. Finally, U.S. Pat. No. 3,731,986, inthe name of James L. Ferguson, entitled "Display Devices UtilizingLiquid Crystal Light Modulation," and assigned to International LiquidXtal Company, disclose a bistable device utilizing nematic liquidcrystal such that the long axis of the nematic material is oriented in ahelical manner from the first electrode to the second electrode. Uponthe application of a threshold voltage, the structure will untwist. Theuse of positive dielectric anisotropy materials is taught in thatpatent.

For a variety of reasons none of these effects by itself is directlysuited for the present application of the AC light valve. To obtain theperformance characteristics that will be described below we havedeveloped a hybrid field effect mode -- -- one that uses a modificationof the conventional twisted nematic effect in the off-state (no voltageon the liquid crystal) and the pure optical birefringence effect of theliquid crystal in the on-state (voltage on the liquid crystal).

To implement this hybrid electro-optic effect, we fabricate the liquidcrystal layer in a twisted alignment configuration; the liquid crystalmolecules at the electrodes are aligned with their long axes parallel tothe electrode surfaces. In addition, they are aligned to lie parallel toeach other along a preferred direction that is fabricated into eachelectrode surface of the device. The twisted alignment configuration isobtained by orienting the preferred directions on the two parallelopposed electrode surfaces such that the respective projections of thefirst and second directions associated respectively with the first andsecond electrodes in any plane which is between and parallel to bothsaid electrode planes intersect at an acute angle for producing atwisted alignment of the axes of the nematic liquid crystal molecules asone progresses along the perpendicular axes to the two electrodes. Thatis to say, as may be best seen in FIG. 5b, the direction of the arrow 30in the plane of electrode 2a represents the alignment direction formolecules such as the molecules having axes 32 immediately adjacent andparallel to the electrode 2a whereas the arrow 31 in the plane ofelectrode 2 represents the direction of alignment for molecules such asthe molecules having axes 33 which are immediately adjacent and parallelto the plane of electrode 2. The intermediate molecules have their axesgradually rotated about imaginary line or axis 34 which is perpendicularto electrode 2 and 2a from the angular position of arrow 30 to therelative angular position of arrow 31 as one progresses across theliquid crystal layer from electrode 2 to electrode 2a. It will be notedthat, purely for convenience of illustration, the relative positions ofthe electrodes 2 and 2a have been reversed as between left and right inthe diagramatic views at FIGS. 5a, 5b, 6a, and 6b from the structuralshowing of FIG. 1. It will also be understood, of course, that FIGS. 5band 6b, are broken diagramatic views which show on an enlarged scaleonly one small cross sectional element of the homogeneous liquid crystallayer 13 shown in cross section in FIGS. 5a and 6a.

In the conventional twisted nematic effect devices mentioned above, theangle 36 between the projection of arrows 30 and 31, or, more precisely,the angle between the projections of these arrows in any common planeparallel to the electrodes in which the two lines or directions wouldintersect, has been 90°. In the present hybrid device it will beexplained below why it is necessary to use an acute angle which, in apreferred device, is substantially 45°. As may be seen in FIG. 5b,molecules in the bulk of the liquid crystal layer rotate through thisangle between the two direction arrows 30 and 31 in traversing the spacebetween the electrodes 2 and 2a. This twisted alignment configuration,combined with the intrinsic optical birefringence of the liquid crystal,causes the polarization direction of linearly polarized incident lightto rotate exactly through the twist angle. This is the so-called twistednematic effect. In conventional twisted nematic devices the twist angleis 90°. As described below, in the device under discussion here, wetwist the molecules through 45°.

In order to introduce these preferred homogeneous alignment directionsany prior art alignment technique may be used. We have, for example,found it advantageous to introduce a preferred direction by preparingthe substrate surfaces in contact with the liquid crystal material inaccordance with previously developed alignment techniques such asshallow angle ion beam etching. Grazing angle deposition of thepassivating layers 3 and 3a can also be used as can mechanical rubbingin the manner taught by U.S. Pat. No. 3,625,591 issued in the name ofMarvin J. Freiser and Ivan Holler, entitled "Liquid Crystal DisplayElement" and assigned to IBM Corporation. However, the particular methodused for obtaining the desired alignment does not per se form a part ofthe present invention which is directed rather to a data processingsystem using a reflection light valve employing the hybrid electro-opticeffect.

To understand the operation of the hybrid field effect mode, firstconsider the off-state. As shown in FIG. 5a, we place a crossedpolarizer/analyer pair, 40 and 41 respectively, between the light valve15 and the source of the unpolarized readout light 9. The polarizer 40is placed in the incident beam and the analyzer 41 is placed in thereflected beam from dielectric mirror 5 which is here shown separatelyfor convenience of illustration. This provides a dark off-state, becauseafter the first pass through the liquid crystal layer 13 the directionof polarization of the linearly polarized incident light 9a is rotatedby 45° as shown at 9b. But upon reflection from the dielectric mirror 5,the light passes a second time through the liquid crystal and itspolarization as shown at 9c is rotated back to the direction of theincident light, where it is blocked by the crossed analyzer 41. Thus thelow transmission of the off-state of the device is determined entirelyby the twisted nematic effect.

The on-state shown diagramatically in FIGS. 6a and 6b is morecomplicated. If we apply voltage and rotate the molecules to thehomeotropic alignment in which the long axes of the molecules areoriented perpendicular to the electrode surfaces and parallel to thedirection of light propagation, the polarization of the light would beunaffected by the liquid crystal and we would have a dark on-state aswell. This would be of no value. Closer scrutiny of the process wherebythe molecules untwist, however, show that between full "off" and full"on" there exists a voltage regime where the device will transmit light.As the voltage is applied to the liquid crystal the molecules begin totilt toward the normal to the electrode surface as illustrateddiagrammatically for molecules 36 in FIGS. 6a and 6b. In thisorientation of the molecules, between parallel and perpendicular, theoptical birefringence of the molecules can affect the polarization ofthe light. As a result, at these intermediate voltages the light thatemerges from the device after reflection from the mirror 5 becomeselliptically polarized, so that some transmission can occur. Thequestion is, how much?

To answer the question, let us consider the orientation of the moleculesas a function of position across the thickness d of layer 13 withvoltage applied to the device. FIGS. 7a and 7b respectively showcalculated values for the twist angle and for the tilt angle of themolecules as a function of position within the liquid crystal layer fora twisted alignment configuration device. By "twist angle" is here meantthe angle of rotation from the vertical about axis 34 in FIGS. 5b and6b. By "tilt angle" is here meant the angle of rotation from the planedefined by the vertical shown by arrow 35a and axis 35 in FIG. 6b. Inboth FIGS. 7a and 7b the position of the molecule as one traverses thedistance d across the liquid crystal between the two electrodes 2 and 2ais plotted horizontally along the X axis, the point midway between thetwo electrodes being shown as d/2. In FIG. 7a the Y axis shows twistangle so that this angle is plotted as a function of the distance. InFIG. 7b the Y axis shows tilt angle so that FIG. 7b is a plot of tiltangle as a function of distance. As shown in FIG. 7a, the effect of thevoltage is to destroy the twist spiral. In the ideal case, with voltageon, half the molecules in the layer adopt the preferred alignmentdirection associated with one electrode and the other half adopt thealignment direction associated with the other electrode. The "voltageon" case is indicated by the uniformly dashed curved line. The "voltageoff" case is indicated by the long and short dash line. The ideal casein which the molecules divide half and half is indicated by the solidvertical line.

There is a realizable voltage regime in which the practical twist angledistribution of a twisted nematic device is close to this idealdistribution. The physical explanation for this behavior is believed tobe as follows. The twist of the molecules is transmitted from layer tolayer by means of "long" range intermolecular alignment forces that areinherent in the liquid crystal. Generally speaking, as the tilt angle ofthe molecules grows (towards the perpendicular), the transmittance ofthe twist, from layer to layer becomes less effective. If any layer hasmolecules aligned perpendicular to the electrodes (parallel to axis 34),the transmittance of the twist by that layer goes to zero. This has theeffect of cutting the entire twist spiral into two separate parts. Whenthis happens the molecules snap into an alignment orientation that isdetermined by the closest electrode. This in turn causes the twist angledistribution to transform to the ideal one shown by the solid verticalline in FIG. 7a. The foregoing describes the nature of the twistmechanism.

Next, consider the effect of the voltage on the twist of an actualdevice. The calculated tilt angle (θ) as a function of position alongthe cell thickness is shown in FIG. 7b. Close to the electrodes the tiltangle is small; but at the center of the layer it is large, becausethere the tilt angle is the sum of the tilts of all the moleculesbetween the electrode surface and the center of the layer. For voltagesthat are just twice the threshold voltage, the tilt angle at the centerof the cell is already 80°. Thus with relatively low voltages switchedto the liquid crystal, the spiral can be snapped and the distribution oftwist angle will be close to the ideal shown in FIG. 7a. Moreover inthis near/ideal state the average tilt angle is much less than 90°. Thedevice takes advantage of the birefringence of this state in thefollowing manner.

The polarization of the light entering the device must be aligned alongthe preferred alignment direction of the entrance electrode (which inthe drawings is the electrode 2) in order to make the twisted nematicoff-state work. Thus when the molecules untwist, the polarization of thelight would be either at 0° or at 90° (in a 90° twist cell) with respectto the majority of the liquid crystal molecules and the netbirefringence effect experienced by the light that passes through thelayer would be very small. Given this picture, the way to maximize thebirefringence (at least to first order) is to orient the preferreddirections of the two electrodes at an angle of 45° with respect to eachother. In this way the polarization of the light will make an angle of45° with respect to the extra-ordinary axis of the liquid crystalthroughout half of the layer. This optimizes the transmission of thedevice.

In FIG. 8 there is shown an experimental curve that characterizes thehybrid field effect liquid crystal device. Curve A is for a 45° twistangle device in accordance with the present invention. Curve B is for aconventional 90° twist angle device. In FIG. 8 the X axis is used toplot rms volts whereas the Y axis is used to plot transmission percent.The data were taken with two micrometer thick, reflection mode cellsfilled with an ester nematic liquid crystal. The polarizer was orientedparallel to the liquid crystal optical axis on the front electrode andthe analyzer was oriented perpendicular to the polarizer. The readoutbeam 9 was from a helium-neon laser. As expected from the abovereasoning, the birefringence of the 45° cell is considerably strongerthan that of the 90° cell so that the maximum transmission for the 45°cell is much larger than for the 90° cell.

Consider now the characteristic curve of this 45° device shown in FIG.8. The curve is characterized by low operating voltage (that is,voltages below 4 volts), by the steep and linear change of thetransmission as a function of the applied voltage, and by the highon-state transmission of 86 percent. These features, combined with thefast response time and the low off-state transmission, provide uniquecharacteristics of the liquid crystal hybrid field effect mode lightvalve.

The sensitometry curve shown in FIG. 10 is a plot of the input lightintensity as abscissa versus an ordinate showing percent transmissionfor the light valve when used in a conventional 500 watt Xenon arcreflection mode projection system for use where high intensityprojection light levels are needed to improve signal-two-noise therebyto assure accurate measurement. The Xenon arc lamp supplied theprojection light 9 schematically indicated in FIG. 1. The illuminationlight 10 was filtered to simulate a P-1 phosphor; it had a centralwavelength of 527.5 nm and a 50% bandwidth of 23.3 nm. The read-out orprojection light 9 was filtered to a narow spectral band centered at 615nm to approximate Helium-neon laser emission for the purpose of thesemeasurements. A conventional photodiode radiometric detector was used.The bias voltage applied to the light valve was 6 volts rms at 20 kHz.The rms current was measured to be 5 milliampere. To obtain the datashown in FIG. 10, we fixed and measured the light incident on thephotoconductor and then we measured the read-out light transmitted fromthe device to the screen. These data represent one point on the curve inFIG. 11.

As can be seen from the figure, the threshold sensitivity occurs atabout 3.3 μW/cm². If we use 10 millisecond for the excitation timeresponse, we find that the threshold exposure for the device is 0.33ergs/cm². We attribute this low input light requirement at the thresholdin this particular device to the design of the CdS photoconductor thatpermits the main part of the input light to be absorbed in the vicinityof the heterojunction. This particular design is set forth in the abovenoted application Ser. No. 625,331. This photoconductor design, however,does not form a part of the present invention since a photoconductor ofthe type shown in Beard U.S. Pat. No. 3,824,002 may be used where lowlevel response is not required. Typically we operate the device at avoltage such that 100 microwatts per square centimeter is the peak inputintensity. This corresponds to a maximum contrast of 90:1 for themeasured device and a sensitivity of 10 ergs/cm² at maximum contrast.

In the present configuration and at its present level of development,the hybrid field effect light valve has been found to have the followingperformance characteristics:

Aperature Size: -- 1 in.²

Sensitivity

(Full Contrast): -- 160 μw/cm² at 525 nm

Resolution: -- 60 lines/mm at 50% MTF

Contrast: -- >100:1

Grayscales: -- 9

Speed:

Excitation (0 to 90%) -- 10 msec

Extinction (100% to 10%) -- 15 msec

Projection Light

Throughput: -- >100 mW/cm²

Reflectivity: -- >90%

Optical Quality: -- <2 wavelength curvature at 6328A

Voltage: -- 6 V_(rms) at 10 kHz ea

The optical data processing capability of the device was established byseveral experiments using the light valve in the system shown in FIG. 9.This system of FIG. 9 is an improvement over the type of system shown inU.S. Pat. No. 3,744,879 issued to Terry D. Beard on July 10, 1973 andoperates on the same general principles as are set forth therein and inthe CODP literature cited above, with the following differences. Inparticular the Beard system uses a transmissive dynamic scattering lightvalve whereas the system of the present invention is designed to use areflective hybrid field effect mode light valve of the type abovedescribed. In FIG. 9 the light valve 15 which is connected to voltagesource 16 is provided with input illumination 10 which is derived from anoncoherent light source 50 supplying light through an inputtransparency 51 containing signal information which is modulated ontothe beam 10 which is then supplied through projection lens 52 andshutter 53 to the input side of the light valve 15.

The projection light 9 is derived from a helium neon laser 54 whichsupplies output through a spatial filter 55 and a recollimating lens 56to a beam splitter 57. Beam splitter 57 sends a portion of theprojection light into the light valve 15 where it is reflected by thedielectric mirror 5 and modulated in accordance with the operation ofthe light valve described above. Output from the light valve 15 istransmitted through beam splitter 57, through a Fourier transform lens58, an analyer 59, and thence to a silicon diode detector 61 on screen60. The portion of the beam 9 from laser 54 which is not initiallyreflected by beam splitter 57 is transmitted straight through to element62 which may be an absorber or which optionally may be an interferometermirror.

As noted above, the system of FIG. 9 operates in accordance with thesame basic principles as are outlined in the Beard U.S. Pat. No.3,744,879 to perform optical data processing, but its response is muchfaster. By way of example, the system has been used in the followingexperiments. The system can be made to reverse the contrast of an imagesimply by changing the voltage applied to the light valve. Afterrotating the analyer associated with the light valve 15 as shown inFIGS. 6a and 6b (and not the otherwise separately shown in FIG. 9) fromits usual perpendicular position to an angle of 22.5° to theperpendicular, only a small change of light valve bias voltage (from 5.5volts rms to 3.6 volts rms for the case illustrated) is required toreverse the contrast of the projected image. That is to say, aphotographic negative can be transformed into a positive and vice-versa.Voltages between these two extremes produce intermediate results. Theintermediate states can be used for level slicing in data reductionschemes.

Another way to manipulate the contrast and the gray levels is to varythe frequency of the power supply. This changes the threshold and theslope of the response of the liquid crystal. Using this feature one can"zoom" the gray scale response of the device to examine in detail theproportions of the input image gray scale distribution. This can be asimple and useful tool for the analysis and data reduction of compleximages derived for example, from visible light, infra-red or x-rayphotography.

The system has numerous other uses. The traditional Fourier transformoperations of CODP are facilitated by the use of a reflection lightvalve such as disclosed. The differentiation or edge enhancement of animage is also possible using known techniques. The use of the presentlight valve in such known techniques makes it possible to perform all ofthese operations in real time in view of the improved response of thelight valve. This is the real significance of the system of the presentinvention. It offers high optical performance in real-time in systemsthat use spatially coherent light.

What is claimed is:
 1. A coherent optical data processor comprising:a. reflective mode alternating current driven liquid crystal light valve means of the hybrid field effect mode type for modulating the polarization state of coherent projection light responsively to a noncoherent input light signal; b. means for providing a noncoherent input light image signal to the input side of said light valve; c. means for providing coherent projection light to the output side of said light valve said means comprising a source of polarized light, said light valve means including analyzer means positioned to block the exit from said light valve of light in a first polarization state and to transmit light from said light valve when its polarization state has been modulated to at least one state different from said first state in said light valve; d. lens means positioned to receive said modulated polarization state light projected from said analyzer of said light valve for effecting a transform of the modulation image in said beam produced by said input signal; and e. means for detecting said transformed image.
 2. Apparatus as in claim 1 wherein said reflective mode alternating current driven liquid crystal light valve of the hybrid field effect type comprise:a. first and second parallel opposed transparent electrode means for application of an alternating electric field across first and second parallel opposed transparent insulating layers which are positioned between and parallel to said first and second electrode means to define in cooperation with an annular spacer member separating said first and second insulating layers an enclosed space containing nematic liquid crystal material, said light valve further having photoconductor means and mirror means positioned between said second transparent electrode and said second transparent insulating layer, said photoconductor means being positioned to receive input writing signal light through said second transparent electrode to spatially and temporally vary the magnitude of said electric field applied across said liquid crystal material as a function of the intensity of said writing light; b. means operatively associated with the first of said transparent insulating layers in the absence of an electric field for aligning the axes of the molecules of a first portion of said liquid crystal material adjacent to said first layer in a first direction in a plane which is parallel to said first layer; c. means operatively associated with the second of said transparent insulating layers in the absence of an electric field for aligning the axes of the molecules of a second portion of said liquid crystal material which is adjacent to said second layer in a second direction in a plane which is parallel to said second layer, the respective orthogonal projections of said first and second directions in any plane which is between and parallel to both said first and second planes intersecting at an acute angle for producing a twisted alignment of the axes of said nematic liquid crystal molecules in the absence of said electric field and a tilted orientation of said axes in the presence of said electric field; d. said means for providing projection light comprising means positioned for providing polarized projection light through said first transparent electrode and said first transparent insulating layer to traverse said liquid crystal material contained between said first and second insulating layers and be reflected by said mirror means to retraverse said liquid crystal material and exit through said first transparent electrode of said light valve, said polarized projection light being subject to changes in polarization state as it traverses said layer of liquid crystal material in both directions, said polarization state being rotated by said twisted nematic alignment in the off state of said field and by the birefringence of said tilted orientation in the on state of said field.
 3. A device as in claim 2 wherein said acute angle between said first and second alignment directions is substantially 45°.
 4. A device as in claim 1 wherein said projection light passes through said first transparent insulating layer orthogonally thereto and along the optic axis of said light valve.
 5. A device as in claim 4 wherein said acute angle between said first and second alignment directions is substantially 45°.
 6. A device as in claim 1 wherein said means for blocking said projection light exiting from said light valve comprises a linear polarizer, the polarization axis of said polarizer making an angle with the axis of polarization of said polarized projection light entering said light valve which is equal to ninety degrees with respect to the polarizing direction of said polarized projection light.
 7. A device as in claim 1 wherein said liquid crystal material has a thickness substantially equal to 2 micrometers.
 8. A device as in claim 1 wherein the thickness of said liquid crystal material has a variation of less than 0.25 micrometers across its surface.
 9. A device as in claim 1 wherein both said means for aligning the axes of the molecules of said liquid crystal material are formed at least in part by grazing angle deposition of said insulating layers respectively in the direction in which molecules adjacent to said layer are to be aligned.
 10. A device as in claim 1 wherein said liquid crystal material is of positive dielectric anisotropy. 